US3419359A - Flame ionization detector - Google Patents

Description

13% 3 1968 N. e. ANDERSON ETAL 3,419,359

FLAME IONIZATION DETECTOR Filed July 8, 1965 INVENTORS. Norman 6. Anderson BY Richard H. Stevens m4 W AIR FLOW ATTORNEY.

United States Patent 3,419,359 FLAME IONIZATION DETECTOR Norman G. Anderson and Richard H. Stevens, Oak

Ridge, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed July 8, 1965, Ser. No. 470,652 4 Claims. (Cl. 23-253) ABSTRACT OF THE DISCLOSURE An improved flame ionization system is provided for the continuous analysis of carbon in submilligram and submicrogram size solid and liquid samples. The sample to be analyzed is deposited on a wire of noble metal which is passed axially through the jet opening of a horizontally fed flame jet assembly. All volatile products from the sample are swept into the combustion zone of the flame by an inert carrier gas flow in close time synchrony with the less volatile components remaining on the wire.

This invention relates to an improved flame ionization system which exhibits an increased sensitivity in determining the carbon content of solid or liquid sample materials during continuous sampling and analyzing procedures.

The determination of the carbon content of naturally occurring materials is of considerable importance in the fields of biology and medicine since carbon is usually present as the common building block of all organic growth processes. Inasmuch as biological materials exist primarily in the liquid and solid phases, a direct and preferably continuous method for analyzing these types of samples is desirable.

Flame ionization systems operate by collecting either the electron or the positive ion current produced by carbon during the combustion of gaseous organic material in a hydrogen or carbon monoxide flame. The electrical signal obtained from the combustion of a given sample is, within wide limits, a linear function of the rate of carbon introduction to the flame.

Prior art flame ionization systems for the continuous analysis of liquid and solid samples have been generally characterized by low detection efliciencies as a result of sample losses from the moving sample carrier as it approached the flame. The losses occurred from the sample carrier by evaporation or pyrolysis as the sample approached the flame and became heated before entering the flame envelope.

It is, accordingly, 2. general object of the invention to provide a flame ionization system which may be used to determine the carbon content of liquid and solid sample materials.

Another object of the invention is to provide a flame ionization system which can be used to directly and continuously measure the carbon content of solid and liquid sample materials.

Another object of the invention is to provide a flame ionization system wherein sample loss prior to flame ionization is minimized.

Still another object of the invention is to provide a flame ionization system having an increased sensitivity for continuous analysis type systems.

Other objects of the invention will become apparent from an examination of the following description of the invention and the appended drawings wherein:

F G. 1 is a schematic diagram of a flame ionization system made in accordance with the present invention and specifically for depositing a liquid sample on the sample transporting wire;

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FIG. 3 is an enlarged, cross-sectional, schematic view of the jet assembly used in the of FIG. 1.

In accordance with the present invention, an improved flame ionization system is provided for the continuous analysis of carbon in both submilligram and submicrogram size solid and liquid samples. The analyzer introduces the samples directly into the flame on a slowly moving loop of noble metal wire which passes along the axis of a horizontally fed flame jet assembly, thereby allowing the sample carrier wire to pass through the jet opening itself and outwardly along the flame axis. All volatile products from the sample are swept into the combustion zone of the flame by an inert carrier gas flow, such as nitrogen, in close time synchrony with the less volatile components remaining on the wire.

In order to facilitate an understanding of the invention, reference is first made to FIG. 1 wherein a flame ionization system for the analysis of dissolved or suspended particulate matter is illustrated. A flame detector chamber 1 is provided with a jet assembly 2 having hydrogen and nitrogen inlets 3 and 4, respectively, an igniter electrode 5, and an electrically biased ion or electron collecting electrode 6 in the form of a wire grid. Although hydrogen is generally referred to herein, other fuel gases such as carbon monoxide may be used which are capable of ionizing sample materials through combustion.

As shown in FIG. 1, the collector electrode 6 is insulated from the detector chamber and is connected through a 300-volt battery 7 to a highly sensitive output circuit 8 for recording and analyzing the ion current pulses from the detector chamber. The igniter 5 is grounded and is located outside of the ion and electron collecting field to avoid competitive collection. With this arrangement, the collector electrode 6 is essentially at +300 volts potential, and operates as a collector for the negative ions and electrons produced by burning a carbonaceous material in the flame 11 discharged from jet assembly 2.

As shown, a wire sample carrier 12 passes axially through jet assembly 2, flame 11, and collector electrode 6 before passing out of the detector chamber through opening 13. Carrier wire 12 forms an endless loop which moves counterclockwise on three pulleys 14. The carrier wire is maintained under tension by means of a tension spring which pulls the lower pulley downward. Two sets of guides 15 are used to accurately position the carrier wire as it passes through jet assembly 2. Carrier wire 12, pulley-s 14, and guides 15 are grounded. Any suitable drive means (not shown) may be provided to drive one of the pulleys to move carrier wire 12 in the desired direction. A hydrogen wire-cleaning flame 16 is positioned to burn background from the carrier wire before it passes through a sample loading zone 17 where a sample is deposited on the wire. The background material is picked up from the pulleys or from the air as dust particles during its passage around the loop.

In the arrangement of FIG. 1, the power supply 7 is 300 volts so that the collector electrode 6 is normally at +300 volts with respect to ground, and acts to maximize negative ion and electron capture on its surface. If desired, the battery voltage can be reversed in polarity to collect the positive current produced simultaneously with the negative-ion-and-electron current during the sample combustion.

To rapidly obtain an analysis of each of a series of samples, a syringe 24 may be used to deposit samples on the wire by hanging one-microliter (1O liter)-size droplets of sample-containing solution from the wire. The

flame ionization system solvent for the sample is of a volatile type, such as carbon tetrachloride, and evaporates to leave the sample residue on the wire. The solvent must be sufficiently pure so that the carbon content of any residue from the nonvolatile portion of it is small in comparison to the sample carbon content. Another means of depositing sample continuously onto the wire utilizes a drop of sample-containing solution, either as supported on a Teflon V-block, or suspended from the end" of a liquid chromatography column where it is desired to sample the liquid flowed from the column and thus monitor the carbon content of the flowing stream. For this arrangement, the drying oven 23 is placed below the wire to drive off solvent prior to entry of the samples into the jet assembly 2. Also, particulate matter suspended in a liquid medium, for example, individual biological cells such as tetr-ahymena pyriformis, can be deposited onto the wire individually by passing the wire through a drop of suitably populated liquid. The particles are dragged out by the wire and the thin film of solution evaporates almost immediately in the case of water. The samples are then conveyed by the wire through the jet assembly 2 and into the flame 11 at a rate which ensures complete combustion of the sample.

FIG. 2 illustrates an electrostatic precipitator 19 for causing airborne particulates to gather on carrier wire 12. Precipitator 19 would replace oven 23 and syringe 24 in loading zone 17 where the sampling of airborne particulate matter is desired. Particulate laden air enters cylindrical collecting chamber 20 through a. Teflon tube 21 which contains a metal ring 22 for creating a corona discharge between it and the wire 12, and thus charging the particulate matter in the air as it flows through the ring. The particulate matter electrostatically collects on carrier wire 12 and remains there as it passes axially through collecting chamber 20 before entering jet assembly 2.

Because of the polarities of electrostatic field required to keep the charged particulates on the wire, the collecting chamber 20 and the jet assembly 2 would necessarilly have to be maintained at several volts negative with respect to the wire, e,g., volts. This is not shown in FIGS. 1 and 2, but could be easily accomplished by batteries or other means without disturbing the collection efliciency of the ionization chamber 1.

Since the ionization current resulting from combustion is a function of the amount of carbon in the sample, the electrical charge of the ion current pulse from the detector is also a function of the carbon content.

An alternative embodiment of the flame ionization system already described would provide an arrangement to collect both the positive-ion and the negative-ion-andelectron currents simultaneously. The two currents could be compared in determining the presence of positive ions of elements formed by surface ionization on the wire in the flame. A suitable arrangement to accomplish simultaneous current collections would involve the use of two collectors operated at +300 and 300 volts D.C., respectively, from a nearly neutral or electrically grounded carrier wire 12 and flame jet assembly 2. In this case, two input circuits would be required.

The jet assembly 2 is shown in an enlarged cross-sectional schematic view in FIG. 3. The jet assembly comprises a cylindrical body portion 25, provided with an axial bore 25 of relatively small diameter compared to the cylindrical body. As shown, the normal operating position of carrier wire 12 within cylinder 25 is along the axis of bore 26. Cylinder 25 is provided with a radial passage 27 connecting the axial bore 26 with the inert carrier gas inlet tube 4.

The exit face of cylinder 25 is provided with an annular recess 28 which is concentric with axial bore 26. An annular gas discharge nozzle 29 is mounted to the cylinder, with its base fitted in groove 28. As shown, the inside wall of the nozzle 29 is spaced from the inner wall and bottom of recess 28 so as to provide an annulus 30 therebetween. A passageway 31 is provided in the wall of the 4 cylinder to connect the annulus with hydrogen inlet tube 3.

The cylinder and nozzle assembly 25 is divided in half lengthwise with one-half being rigidly attached to a fixed portion 32 of a metal mounting fixture 33 as illustrated in FIG. 1. The other half of the assembly is rigidly fastened to a pivotally mounted portion 34 of the fixture which permits the halves of the assembly to be swung apart to a position where a length of the carrier wire 12 can be positioned between them. The two halves are then swung into mating position and locked. The metal mounting fixture '33 is designed to act as a heat sink so as to prevent excessive temperatures from developing in cylinder 25. Such high temperatures would cause premature vaporization of the sample on carrier wire 12 prior to its passage through the combustion zone of flame 11.

In a typical operation of the jet assembly 2, hydrogen and nitrogen are admitted continuously to the assembly through inlet tubes 3 and 4, respectively. After ignition, the hydrogen issuing from the nozzle 29 forms a sheath of flame 11 which encompasses the gas and carrier wire passing through the nozzle. This arrangement ensures combustion of virtually all of the carbon present in the sample entering the flame, whether the sample is in the solid state as particulate matter on the wire or in the gaseous state mixed with the nitrogen carrier gas which passes along bore 26.

In comparing the present embodiment (jet assembly) with a prior art system wherein an exposed wire was simply drawn transversely through a flame, a sensitivity gain of at least 127 was achieved using the jet assembly. The sample material used was chrysene (C H M.P. 250 C.) having calculated carbon contents from 10 to 10' grams per sample particle. The calculated sensitivity of the prior art system was 0.000037 coulomb/ gram carbon, while the calculated sensitivity of the present jet assembly was at least 0.00472 coulomb/ gram carbon. This increase in sensitivity appears directly attributable to the elimination of sample loss due to precombustion evaporation in the jet assembly plus the eflect of the carrier gas in sweeping that sample portion which was vaporized into the combustion zone of the flame in close time synchrony with the remaining unvaporized sample.

Another application of the jet assembly would be in the case of atomic absorption spectrophometric analysis for trace concentrations of metals in very small samples. Ordinarily, samples to be analyzed are dissolved in water or other solvent and sprayed as an aerosol into a flame to determine the chromatic absorption of the vaporized metal at its own characteristic wavelength. A faster and simplified sample handling technique, permitting continuous or highly repetitive batch sampling type operation, would involve the direct introduction of solid' or undiluted liquid samples into the flame on a sample transport means such as the wire described above. Since the precombustion evaporation problem would also exist in this instance, the jet assembly 2 could be used to conserve and direct the entire sample into the flame in the same general manner as accomplished for flame ionization analysis. The undiluted sample would also provide a concentrating effect for the contaminant sought in the flame, potentially making the analytical method more sensitive.

The above description of the invention was offered for illustrative purposes only and should not be interpreted in a limiting sense. It is intended that the invention be limited only by the claims appended hereto.

What is claimed is:

1. An improved flame jet assembly for the flame ionization of carbonaceous samples on a moving wire comprising: a cylindrical body portion having an axial bore for the passage of said wire, an annular recess located concentrically about said axial bore at the flame discharge end of said body portion, said axial bore being of a relatively small diameter in comparison with the diameter of said body portion, means for supplying an inert carrier gas to said bore, and means for supplying a fuel gas to said recess; an annular gas discharge nozzle engaging the outside wall of said recess, said fuel gas from said recess and said inert carrier gas from said bore mixing in said nozzle before discharging therefrom; and means for supporting and removing heat from said body portion.

2. An improved flame jet assembly for the flame ionization of carbonaceous samples on a moving wire comprising: a cylindrical body portion having an axial bore for the passage of said wire, first and second gas passageways, and an annular recess located concentrically about said axial bore at the flame discharge end of said body portion, said first passageway communicating between said axial bore and the exterior surface of said body portion for the conveyance of an inert carrier gas to said axial bore, said second passageway communicating between the exterior surface of said body portion and the bottom surface of said annular recess for the conveyance of a fuel gas to said recess; and an annular gas discharge nozzle sealably engaging a portion of the outer cylindrical surface of said annular recess while being spaced apart from its inner cylindrical surface and its bottom surface, said fuel gas from said recess and said inert carrier gas from said axial bore mixing together in said nozzle and then discharging therefrom.

3. An improved flame jet assembly for the flame ionization of carbonaceous samples on a moving Wire comprising: a horizontally oriented cylindrical body portion having an axial bore for the passage of said wire, first and second gas passageways, and an annular recess located concentrically about said axial bore at the flame discharge end of said body portion, said axial bore being of a small diameter relative to the diameter of said body portion, said first passageway communicating between said axial bore and the exterior surface of said body portion for the conveyance of an inert carrier gas to said bore, said second passageway communicating between the exterior surface of said body portion and the bottom surface of said annular recess for supplying a fuel gas to said recess; an annular gas discharge nozzle sealably engaging a portion of the outer cylindrical surface of said annular recess while being spaced apart from its inner cylindrical surface and its bottom surface, said fuel gas from said recess and said inert carrier gas from said axial bore mixing together in said nozzle and then discharging therefrom; and a massive heat-conducting support member engaging said body portion so as to support and remove heat therefrom.

4. An improved flame jet assembly for the flame ionization of carbonaceous samples on a moving wire comprising: a horizontally oriented cylindrical body portion having an axial bore for the passage of said wire, first and second gas passageways, and an annular recess located concentrically about said axial bore at the flame discharge end of said body portion, said axial bore being of a small diameter relative to the diameter of said body portion, said first passageway communicating between said axial bore and the exterior surface of said body portion for the conveyance of an inert carrier gas to said bore, said second passageway communicating between the exterior surface of said body portion and the bottom surface of said annular recess for the conveyance of a fuel gas to said recess; an annular gas discharge nozzle sealably engaging a portion of the outer cylindrical surface of said annular recess while being spaced apart from its inner cylindrical surface and its bottom surface, said fuel gas from said recess and said inert carrier gas from said axial bore mixing together in said nozzle and then discharging therefrom; said body portion and nozzle being split longitudinally; and a massive, heat-conducting, hinged support member engaging said split body portions so as to permit said split body portions to be moved apart for the insertion of said Wire and then closed for operation of said flame jet assembly.

References Cited UNITED STATES PATENTS 3,128,619 4/1964 Lieberman 23-255 XR 3,141,741 7/1964 Hoel et a1. 23253 3,292,420 12/1966 Scott 23232 XR OTHER REFERENCES James et al.: Chem. and 1nd, London, May 2, 1964, pp. 746-748.

Stoutfer et al.: Biochim. et Biophys. Acta 93, 191-193 (1964).

MORRIS O. WOLK, Primary Examiner.

R. M. REESE, Assistant Examiner.

US Cl. X.R. 23255, 232

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